Depth and length: fathoms, metres and feet
Explainer · Last reviewed 2026-08-14
A fathom is exactly six feet, which makes fathoms, feet and metres interconvertible without error — and makes this one of the very few things in fishing that can be calculated rather than estimated.
THREE UNITS ARE IN SIMULTANEOUS USE FOR DEPTH AND LENGTH IN FISHING, and a reader will meet all three in one conversation on one boat.
THE FATHOM is defined as exactly six international feet, which is exactly 1.8288 metres. It came from the span of outstretched arms and survived because it is the length of a hauled armful of line, which is how depth was measured by hand for centuries. Charted depths in older British and American practice, wreck depths, and a great deal of boat-fishing conversation are still in fathoms.
THE FOOT AND THE METRE need no explanation except that the international foot is defined as exactly 0.3048 metres, which is what makes every conversion here exact rather than approximate.
WHY THE EXACTNESS MATTERS. This platform refuses to publish calculators whose outputs carry uncertainty that is not shown. A conversion between defined units carries none: it is a multiplication by a constant fixed by definition, the result is exact, and there is no way for it to be quietly wrong. That is the entire reason the unit converter exists and the other tools on the roadmap do not.
WHERE YOU WILL MEET THE MIXTURE.
CHARTS. Older charts are in fathoms, modern ones in metres, and some in both with the units stated in the margin. Reading a depth off a chart without checking which is a genuine and occasionally serious error.
SOUNDERS AND ELECTRONICS, which are switchable and are frequently left on whatever the previous owner set.
ROD LENGTHS, sold in feet in Britain and America and in metres across most of Europe.
LINE, sold by the metre and by the yard.
AND THE THING A CONVERSION WILL NOT TELL YOU, which is the important caveat. A CHARTED DEPTH IS NOT THE DEPTH UNDER YOUR BOAT. Charted soundings are given relative to a stated datum — usually a low-water reference — and the actual depth at any moment is that figure plus the height of tide. Converting the number changes the units and does not change what it is measured from. On a coast with a large tidal range the difference between the charted figure and the water under the hull can be several metres, and it is the whole reason tidal height matters to anybody navigating.
SO THE CONVERSION IS EXACT AND THE INTERPRETATION IS NOT, and knowing which half is which is the useful part.
Fathoms, marks and the units a chart uses that a rod does not
Unit conversion in fishing is not only a metric-and-imperial problem. Several of the units an angler meets belong to navigation and survey rather than to tackle, and they are the ones that cause genuine confusion.
THE FATHOM IS A DEPTH UNIT AND IT SURVIVES BECAUSE OF HOW IT WAS MEASURED. Six feet is roughly the span of a person's outstretched arms, so a leadline was hauled and counted in arm-spans. That is why depths were reported in fathoms for centuries and why the unit persists in traditional fishing vocabulary and in older charts long after modern charts moved to metres.
WHICH MEANS TWO CHARTS OF THE SAME GROUND CAN DISAGREE BY A FACTOR OF NEARLY TWO without either being wrong. A chart in fathoms and a plotter in metres describing the same bank are the standard source of a serious misreading, and it is worth checking which unit a depth is in before acting on it.
ROD LENGTHS ARE STILL SOLD IN FEET IN BRITAIN AND IN METRES ALMOST EVERYWHERE ELSE, and the two conventions coexist in the same tackle shop. A twelve-foot rod and a 3.6-metre rod are the same object described by two markets.
LINE DIAMETER IS METRIC ALMOST UNIVERSALLY and line strength almost never is, which produces the peculiar situation of a spool labelled in millimetres and pounds at the same time.
AND SEA DISTANCE IS NAUTICAL RATHER THAN STATUTE. A nautical mile is one minute of latitude, which is why it exists and why it is not the same as a land mile — a distinction that matters the moment somebody plans a small-craft trip from a road map's sense of distance.
THE CONVERSIONS THEMSELVES ARE EXACT MULTIPLICATIONS BY A DEFINED CONSTANT, which is why FishingHQ builds a tool for them and refuses to build one for anything requiring a fitted parameter. There is no assumption in a conversion and no way for it to be quietly wrong.
Practitioner consensus
Widely and independently recommended by experienced anglers and specialist sources. This is real angling knowledge, not proof: controlled evidence for it is limited, and FishingHQ records it as convention rather than as a demonstrated effect.
- Sea Angler — Bauer Media (Specialist angling publication)Relied on for: Long-running published description of British sea angling practice, including trace construction, dropper loops and shock leaders.FishingHQ's interpretation of what it saysCitation checked — the work exists and is described correctly
- General angling website restatements of established technique — Various (Credible secondary source)Relied on for: Widely restated descriptions of established terminal and connection convention, recorded as practitioner consensus rather than as scholarship.Background and mechanism onlyCitation checked — the work exists and is described correctly
Where to go next
Gear this affects
What these connections rest on (1)
Practitioner consensus — 1 connection
Widely and independently recommended by experienced anglers and specialist sources. This is real angling knowledge, not proof: controlled evidence for it is limited, and FishingHQ records it as convention rather than as a demonstrated effect.
Rods
A mechanism edge from a record to the reasoning beneath it, resting on practice reported consistently across independent sources rather than on experiment.
- Angling Times — Bauer Media (Specialist angling publication)Relied on for: Contemporary UK coarse, match and lure angling practice as reported by practitioners, including tackle convention.States this directlyCitation checked — the work exists and is described correctly
- Sea Angler — Bauer Media (Specialist angling publication)Relied on for: Contemporary UK sea angling practice as reported by practitioners, including tackle, bait and lure convention.States this directlyCitation checked — the work exists and is described correctly
Related reading
What these connections rest on (1)
Practitioner consensus — 1 connection
Widely and independently recommended by experienced anglers and specialist sources. This is real angling knowledge, not proof: controlled evidence for it is limited, and FishingHQ records it as convention rather than as a demonstrated effect.
Reading the tide
A mechanism edge from a record to the reasoning beneath it, resting on practice reported consistently across independent sources rather than on experiment.
- Angling Times — Bauer Media (Specialist angling publication)Relied on for: Contemporary UK coarse, match and lure angling practice as reported by practitioners, including tackle convention.States this directlyCitation checked — the work exists and is described correctly
- Sea Angler — Bauer Media (Specialist angling publication)Relied on for: Contemporary UK sea angling practice as reported by practitioners, including tackle, bait and lure convention.States this directlyCitation checked — the work exists and is described correctly
How this compares
What these connections rest on (1)
Practitioner consensus — 1 connection
Widely and independently recommended by experienced anglers and specialist sources. This is real angling knowledge, not proof: controlled evidence for it is limited, and FishingHQ records it as convention rather than as a demonstrated effect.
What a tide table is telling you
Two records a reader is deciding between, joined so that the trade is visible from either side.
- Angling Times — Bauer Media (Specialist angling publication)Relied on for: Contemporary UK coarse, match and lure angling practice as reported by practitioners, including tackle convention.States this directlyCitation checked — the work exists and is described correctly
- Sea Angler — Bauer Media (Specialist angling publication)Relied on for: Contemporary UK sea angling practice as reported by practitioners, including tackle, bait and lure convention.States this directlyCitation checked — the work exists and is described correctly
Safety reading
What these connections rest on (1)
Authoritative guidance — 1 connection
Stated by a fisheries authority, scientific institute or recognised angling body. Authoritative, but summarising rather than demonstrating.
Boat safety for anglers
Written because the record already states the hazard in its own prose and carried no safety edge to it. A unit conversion that is a navigation input is a different class of object from one that is a tackle input.
- Met Office shipping forecast, inshore waters forecast and marine warnings — Met Office, United Kingdom (Government fisheries body)Relied on for: Marine forecasting and the units and conventions used in describing sea state, depth and distance to small-craft users.Background and mechanism onlyCitation checked — the work exists and is described correctly
- Sea Angler — Bauer Media (Specialist angling publication)Relied on for: British sea angling practice including tackle convention, terminology and the units the sport is described in.FishingHQ's interpretation of what it saysCitation checked — the work exists and is described correctly
- Angling Times — Bauer Media (Specialist angling publication)Relied on for: Contemporary UK angling practice as reported by practitioners, including tackle comparison and the faults anglers meet.FishingHQ's interpretation of what it saysCitation checked — the work exists and is described correctly
History and culture
What these connections rest on (1)
Practitioner consensus — 1 connection
Widely and independently recommended by experienced anglers and specialist sources. This is real angling knowledge, not proof: controlled evidence for it is limited, and FishingHQ records it as convention rather than as a demonstrated effect.
Handline and dropline traditions
A comparison joined to the history that dissolves it. Written where the two things being compared arrived at different times for different reasons, so that 'which is better' turns out to be the wrong question — and refused where the comparison is genuinely contemporary.
- The Compleat Angler, or the Contemplative Man's Recreation — Richard Marriot, 1653 (Reference work)Relied on for: Seventeenth-century English angling practice including tackle, hooks, lines, baits and artificial flies.States this directlyPassage read and verified
- Fishers' Craft and Lettered Art: Tracts on Fishing from the End of the Middle Ages — University of Toronto Press, 1997 (Scientific literature)Relied on for: Medieval and early modern angling tracts and the development of tackle, technique and the literature describing them.Background and mechanism onlyCitation checked — the work exists and is described correctly
- Freshwater Fishing — Macdonald & Jane's, 1975 (Specialist angling publication)Relied on for: Comprehensive angling practice across disciplines, including the development of tackle and the conventions attached to it.States this directlyCitation checked — the work exists and is described correctly
Sources for this page
- NOAA Ocean Service tides and water levels educational tutorial — National Oceanic and Atmospheric Administration, United States (Government fisheries body)Relied on for: Public education on tidal datums, charted depth reference levels and how predicted water levels relate to charted soundings.States this directlyPassage read and verified
- Sea Angler — Bauer Media (Specialist angling publication)Relied on for: Contemporary UK sea angling practice as reported by practitioners, including tackle vocabulary, lead sizes and depth conventions.States this directlyCitation checked — the work exists and is described correctly
- IGFA International Angling Rules and World Record Requirements — International Game Fish Association (Angling or fisheries organisation)Relied on for: International angling rules including the unit conventions used for line classes and record measurement.States this directlyPassage read and verified
How FishingHQ labels evidence is explained in the evidence policy.